The function of a bonding agent in an ammonium perchlorate oxidized solid propellant is to adequately coat the ammonium perchlorate (AP) surfaces, a point of disagreement in the propellant field is whether it is desirable for a bonding agent to be soluble in the liquid prebinder. It has been postulated that adequate coating would occur when the bonding agent is dissolved in the prebinder; however, recent experimentations indicated that most bonding agents that are soluble tend to remain dissolved in the binder even after the solid ingredients are added. Some function as a bonding agent still appears to take place in hydroxy-terminated polybutadiene even with solubility of the bonding agent. Better bonding and thus superior mechanical properties and sometimes better aging characteristics have been obtained with highly miscible rather than soluble bonding agent condidates. It is believed that the partial insolubility of such a bonding agent causes it to be driven to the surface of the solids where it is needed to perform its function. At the same time, the partial solubility, miscibility, must be great enough to insure even, adequate coverage of the solids, especially the oxidizer such as ammonium perchlorate, (AP). The bonding agent covering the AP surface subsequently reacts with components in the binder, especially the curing agent so that the encapsulated AP is mechanically and chemically bound to the elastic binder, thus reinforcing the matrix system to give superior physical properties. If the bonding agent is soluble in the binder and there is no force such as a chemical reaction to drive the bonding agent to the surface of the AP, e.g., amine containing bonding agent, much of the bonding agent tends to remain dissolved in the binder and subsequently reacts "in situ" with the binder ingredients, especially the polymer and the curing agent, rather than first encapsulting the AP.
A state of the art bonding agent HX752, a difunctional aziridine, is soluble in both hydroxyterminated polybutadiene (HTPB) and polyester binders at 140.degree. F. In HTPB propellants with proper processing procedures HX752 performs as an adequate bonding agent. It is chosen many times because of availability and ease of incorporation into the propellant. However, when HX752 was used in a polyester binder with AP oxidizer, no bonding occurred.
Since HTPB binders have been more widely used in propellants the problem described above has not come to light because "state-of-the-art bonding agents" function to some degree as bonding agents in HTPB propellant regardless of their solubility relationship in the prebinder. The decrease in bonding agent performance has, perhaps by necessity, been compensated for in HTPB propellants by experimentally adjusting the amount of the bonding agent to achieve the desired strain at maximum stress/break for a required mission.
Recent propellant developments show that some state-of-the-art bonding agents used in HTPB propellants do not function as bonding agents in a polyester binder. Further, bonding agents such as cyanoethyl substituted tetraethylene pentamine (TEPAN) disclosed in U.S. Pat. No. 4,000,023 and the glycidol reaction product of cyanoethyl substituted tetraethylene pentamine (TEPANOL) have been employed as bonding agents for polyurethane containing ammonium perchlorate oxidizing agent. These bonding agents (TEPAN or TEPANOL), in use, produced a disadvantage in that ammonia is liberated during the propellant mix and cure cycles due to displacement of ammonia from ammonium perchlorate (AP) by amine groups in the bonding agent. The amine-ammonium perchlorate reaction required subsequent ammonia removal from the mix because ammonia could consume part of the isocyanate curing agent and thus interfere with the cure and the mechanism for achieving more desirable mechanical properties.
The above problems were obviated by reacting TEPAN or TEPANOL with a selected ammonium oxidizer salt to form an adduct of TEPAN or TEPANOL and selected oxidizer salt. These adducts of selected ammonium oxidizer salts, e.g., ammonium perchlorate (AP), ammonium nitrate (AN), ammonium sulfate (AS) and ammonium formate (AF), and their use in propellants employing HTPB binders are disclosed and claimed in copending patent application Ser. No. 488,249 by Ducote et al., filed June 9, 1983, and now U.S. Pat. No. 4,493,741.
The present invention relates to bonding agents for solid propellants employing polyester binders.
Therefore, an object of this invention is to provide excellent bonding agents for use in propellants employing polyester binders.
A further object of this invention is to provide a bonding agent which has excellent performance as a bonding agent with co-oxidizer ammonium salts in a polyester binder propellant system.